The process of seamless steel tube making is essentially a high temperature hot rolling operation. Conventional seamless tube manufacture comprises the steps of reheating a billet of steel having the desired chemical composition in a reheating furnace to a temperature of about 1,200.degree. to 1,300.degree. C., passing the said billet through a piercing mill wherein the billet is formed into a hollow steel shell, elongating the steel shell in a retained mandrel mill wherein the thickness of the shell wall is reduced, and then reducing the diameter of the elongated shell by stretching the shell in a stretch reducing mill. The resulting steel tubes can then be heat treated to increase the final strength of the finished product. This final heat treating stage, although fairly expensive, has heretofore been required in order to obtain a final product with yield strengths in excess of 70,000 psi.
It is known to be possible to increase the final strength of steel structures by controlled rolling and micro-alloying. See, for example, Canadian Patent No. 1,280,015, Boratto et al., Feb. 12, 1991. Micro-alloyed steels are generally low carbon steels containing columbium and/or titanium at levels totalling approximately 0.05% by weight of the steel or vanadium at levels of about 0.1%. In controlled rolling, an ingot or slab of micro-alloyed steel is first heated to a temperature of about 1,250.degree. C. and then subjected to a rolling schedule involving delays in the pass sequence such that substantial strain is applied to the slab or ingot below a temperature of 950.degree. C. The strain applied in conventional controlled rolling (CCR) causes pancaking of the austenite crystals which, in turn, yields a fine ferrite grain structure upon cooling due to the presence of the distorted austenite grain structure and of ferrite growth retarding alloying agents such as columbium or titanium.
In seamless tube production, however, 70% to 90% of the strain is applied to the billet at a temperature above 1,040.degree. C., which temperature is well above the "no-recrystallization" temperature T.sub.nr for static recrystallization of the steel used (The temperature T.sub.nr is dependent not only upon alloy composition but also upon the rolling schedule. See e.g. Tanaka et al., "Three Stages of the Controlled Rolling Process", Microalloying '75, Union Carbide, 1975, p.107, and also the above mentioned Boratto Canadian Patent No. 1,280,015, where the symbol T.sub.n is used instead of T.sub.nr.) Moreover, once the steel has cooled below the T.sub.nr and is processed in the stretch reducing mill, there is insufficient time between passes for sufficient carbonitride to occur, which is a requirement for pancaking of the austenite. As a result, conventional controlled rolling cannot appreciably increase the yield strength of seamless tubes because no significant austenite pancaking can occur at the desired temperatures.
There are a number of known methods for increasing the yield strength of seamless steel tubes, including recrystallization controlled rolling (RCR), which also produces ferrite grain refinement: See, R. Barbosa, S. Yue, J. J. Jonas and P. J. Hunt, "Recrystallization Controlled Rolling of Seamless Tubing"; Proc. International Conference on Phys. Metall. of Thermomechanical Processing of Steels and Other Metals (Thermec-88), Tokyo, Japan, June 1988, pp. 535-542. In conventional recrystallization controlled rolling, reductions are effected above the T.sub.nr, and the austenite grain size is reduced by static recrystallization after the application of strain. Grain growth of the recrystallized austenite is inhibited by the use of alloying in additions, particularly titanium. Upon cooling, the austenite transforms into ferrite having a fine grain structure and increased yield strength. The yield strength of steel tubes may also be increased by solid solution strengthening and by increasing the volume fraction of the carbon-containing phase, pearlite. Classical alloying additives which increase solid solution strengthening include molybdenum and manganese, whereas an increase in carbon content leads to an increase in pearlite volume fraction.
A further component of strengthening is contributed by precipitation hardening, as caused for example by the formation of fine precipitates of vanadium nitride.
The yield strength of finished seamless steel tubes may also be increased by accelerated cooling of the steel tubes during the austenite-to-ferrite transformation, which imparts a further grain refining effect. Posdena et al in a paper entitled "Application of Microalloyed Steels to The Production of Seamless Line Pipe and OCTG"; Proc. Conference on `HSLA Steels '85`, Beijing, China, November 1985, pp. 493-506, discloses the manufacture of seamless steel tubes, made from microalloyed steels having moderate carbon concentrations (e.g. 0.08%) and microalloying additions such as titanium, vanadium and niobium, which are subjected to accelerated cooling following the stretch reducing mill. However, the steels produced by the above prior art methods are still not strong enough to be used for grades of casing or line pipe requiring yield strength in excess of 70,000 psi, without subsequent quenching and tempering.